The rapidly spinning pulsar -- an extraordinarily dense object created when a massive star exploded as a supernova -- is called J1903+0327 and is located about 21,000 light years from Earth, the astronomers said.
A light year is about 6 trillion miles, the distance light travels in a year.
"The big question is -- how in the heck did this thing form, because it doesn't follow our standard models of how these things form," astronomer Scott Ransom of the National Radio Astronomy Observatory in Charlottesville, Virginia, said in a telephone interview on Thursday.
This object is known as a millisecond pulsar because of its speedy whirl -- it spins on its axis 465 times per second.
"The reason why we're so excited about this is the impact it might have on our understanding of where the pulsars that we look at are coming from," David Champion, an astronomer at Canada's McGill University, told SPACE.com. "We've never seen anything like this before."
Until now, all of the ones found orbiting with another star have been doing so with a white dwarf, another type of dying star. In each case, they shared a perfectly circular orbit. But this one has a very elongated orbit around a star similar in size and composition to our sun.
"What we have found is a millisecond pulsar that is in the wrong kind of orbit around what appears to be the wrong kind of star," astronomer David Champion of the Australia Telescope National Facility said in a statement. "Now we have to figure out how this strange system was produced."
It was detected using a radio telescope in Puerto Rico.
Pulsars are a rare type of neutron star whose strong magnetic fields channel lighthouse-like beams of light and radio waves that whirl around as the star spins.
Typical pulsars spin once a second to about 10 or 20 times a second. But millisecond pulsars spin far more rapidly.
The understanding had been that these started out as typical, slower-spinning pulsars, then built up speed after material expelled from another star reached the pulsar's surface, giving it momentum.
"If you were to ask any astronomer if we would have found a system like this, they would have said no. So this is a very big surprise," Ransom said.
The scientists, writing in the journal Science, speculate a third star -- perhaps a neutron star or white dwarf -- might be orbiting with the other two. Scientists know of about 100 pulsars in two-star, or binary, systems, and this might be the first in a triple-star system, Ransom said.
Incredible Space Pictures
Below: An artist's graphical representation obtained from NASA depicts a pulsar, center, surrounded by whirling beams of light and radio waves. Astronomers are puzzled by the discovery of a pulsar with an unusual orbit never before seen.Below: A pair of spiral galaxies found in the constellation of Hercules meet in a head-on collision in this new image from the Hubble Space Telescope. Fifty-nine new swirling galaxy images from the famed telescope were released on April 24 on the 18th anniversary of its launch.
Above: Even galaxies get bullied. Here, a so-called "death star galaxy" blasts a nearby galaxy with a jet of energy. Scientists said that if this happened in the Milky Way, it would likely destroy all life on Earth.
Below: In February, photos taken in July 2007 were released showing a puzzling haze that brightened and dimmed over Venus' southern latitudes. The haze moved toward the equator and then back to the south pole. Scientists aren't sure what is causing it.
Below: In 2005, NASA's Hubble Telescope captured this image of the Crab Nebula, a six-light-year-wide expanding remnant of a star's supernova explosion. Japanese and Chinese astronomers witnessed this violent event in 1054.